When replying to Outlook.com email addresses that have worked for a long time, the replies are now being bounced. I get the meesage -

Delivery to the following recipients was aborted after 0 second(s):

Reason: The following message to ...@outlook.com was undeliverable.The reason for the problem:  550 5.7.1 Unfortunately, messages from [IP address] weren't sent. Please contact your Internet service provider since part of their network is on our block list (AS3140). You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors.

Having been on the phone to my ISP, they asked that I try webmail, and the email appears to have gotten through. Therefore it's an issue with Windows Live Mail, running on Windows 7 on two different PCs.

How do I fix this?